Selected article

Fusion power is a theoretical form of power generation in which energy will be generated by using nuclear fusion reactions to produce heat for electricity generation. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, and at the same time, they release energy. This is the same process that powers stars like our Sun. Devices designed to harness this energy are known as fusion reactors.

Fusion processes require fuel and a highly confined environment with a high temperature and pressure, to create a plasma in which fusion can occur. In stars, the most common fuel is hydrogen, and gravity creates the high temperature and confinement needed for fusion. Fusion reactors generally use hydrogen isotopes such as deuterium and tritium, which react more easily, and create a confined plasma of millions of degrees using inertial methods (laser) or magnetic methods (tokamak and similar), although many other concepts have been attempted. The major challenges in realising fusion power are to engineer a system that can confine the plasma long enough at high enough temperature and density, for a long term reaction to occur, and for the most common reactions, managing neutrons that are released during the reaction, which over time can degrade many common materials used within the reaction chamber.

1.
Smoke detector
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A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are housed in enclosures, typically shaped like a disk about 150 millimetres in diameter and 25 millimetres thick. Smoke can be detected optically or by physical process, detectors may use either. Sensitive alarms can be used to detect, and thus deter, Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. The US National Fire Protection Association estimates that nearly two-thirds of deaths from home fires occur in properties without working smoke detectors, the first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton, an associate of Thomas Edison. George Andrew Darby patented the first European electrical heat detector in 1902 in Birmingham, in the late 1930s Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. He expected that gas entering the sensor would bind to ionized air molecules and his device did not meet its purpose, small concentrations of gas had no effect on the sensors conductivity. Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current. Smoke particles from his cigarette had apparently done what poison gas could not, jaegers experiment was one of the advances that paved the way for the modern smoke detector. In 1939 Swiss physicist Ernst Meili devised an ionisation chamber device capable of detecting combustible gases in mines and he also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism to a strength sufficient to activate an alarm. Ionisation smoke detectors were first sold in the United States in 1951, they were used only in commercial and industrial facilities in the next several years due to their large size. In 1955 simple home fire detectors for homes were developed, detecting high temperatures, the United States Atomic Energy Commission granted the first license to distribute smoke detectors using radioactive material in 1963. The first low-cost smoke detector for use was developed by Duane D. Pearsall in 1965. The SmokeGard 700 was a beehive-shaped, strong fire-resistant steel unit, the company began mass-producing these units in 1975. Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors, the first single-station smoke detector was invented in 1970 and made public the next year. It was an ionisation detector powered by a single 9-volt battery and they cost about US$125 and sold at a rate of a few hundred thousand per year. The previous alarm horns, which required specialty batteries, were replaced with horns that were more energy-efficient and these detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned for more effective operation. The rechargeable batteries were replaced by a pair of AA batteries along with a plastic shell encasing the detector

2.
Nuclear reactor
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This article is a subarticle of Nuclear power. A nuclear reactor, formerly known as a pile, is a device used to initiate. Nuclear reactors are used at power plants for electricity generation. Heat from nuclear fission is passed to a fluid, which runs through steam turbines. These either drive a ships propellers or turn electrical generators, Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, some are run only for research. As of April 2014, the IAEA reports there are 435 nuclear power reactors in operation, when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission. The heavy nucleus splits into two or more nuclei, releasing kinetic energy, gamma radiation, and free neutrons. A portion of neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons. This is known as a chain reaction. To control such a chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions, commonly-used moderators include regular water, solid graphite and heavy water. Some experimental types of reactor have used beryllium, and hydrocarbons have been suggested as another possibility, the reactor core generates heat in a number of ways, The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms. The reactor absorbs some of the rays produced during fission. Heat is produced by the decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some even after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases approximately three times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — usually water but sometimes a gas or a metal or molten salt — is circulated past the reactor core to absorb the heat that it generates

3.
Sight (device)
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A sight is a device used to assist aligning or aim weapons, surveying instruments, or other items by eye. Sights can be a set or system of markers that have to be aligned together as well as aligned with the target. They can also be optical devices that allow the user to see the image of an aiming point in the same focus as the target. These include telescopic sights and reflector sights, There are also sights that project an aiming point onto the target itself, such as laser sights. At its simplest, a sight typically has two components, front and rear aiming pieces that have to be lined up, sights such as this can be found on many types of devices including weapons, surveying and measuring instruments, and navigational tools. On weapons, these sights are usually formed by rugged metal parts, giving them the iron sights. On many types of weapons they are built-in and may be fixed, adjustable, or marked for elevation, windage, target speed and they are also classified in forms of notch or aperture. Optical sights use optics that give the user an image of an aiming point or pattern superimposed at the same focus as the target. Telescopic sights are used on a range of devices including guns, surveying equipment. These sights have been around for over 100 years and been used on all types of weapons, reflector sights were first used as a weapon sight in German aircraft towards the end of World War I. Collimator sight Holographic weapon sight There are many types of sighting devices and they can be fixed, mechanical, optical, computational, or a mixture of all of these attributes

4.
Nuclear weapon
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A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from small amounts of matter. The first test of a bomb released the same amount of energy as approximately 20,000 tons of TNT. The first thermonuclear bomb test released the same amount of energy as approximately 10 million tons of TNT, a thermonuclear weapon weighing little more than 2,400 pounds can produce an explosive force comparable to the detonation of more than 1.2 million tons of TNT. A nuclear device no larger than traditional bombs can devastate a city by blast, fire. Nuclear weapons are considered weapons of destruction, and their use. Nuclear weapons have been used twice in nuclear warfare, both times by the United States against Japan near the end of World War II, the bombings resulted in the deaths of approximately 200,000 civilians and military personnel from acute injuries sustained from the explosions. The ethics of the bombings and their role in Japans surrender remain the subject of scholarly, since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated on over two thousand occasions for the purposes of testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them, israel is also believed to possess nuclear weapons, though in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium and the Netherlands are nuclear weapons sharing states, south Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons. Modernisation of weapons continues to occur, all existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as bombs or atom bombs. This has long noted as something of a misnomer, as their energy comes from the nucleus of the atom. The latter approach is considered more sophisticated than the former and only the approach can be used if the fissile material is plutonium. A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons of TNT, all fission reactions necessarily generate fission products, the radioactive remains of the atomic nuclei split by the fission reactions. Many fission products are highly radioactive or moderately radioactive. Fission products are the radioactive component of nuclear fallout

5.
Nuclear power
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Nuclear power is the use of nuclear reactions that release nuclear energy to generate heat, which most frequently is then used in steam turbines to produce electricity in a nuclear power plant. The term includes nuclear fission, nuclear decay and nuclear fusion, since all electricity supplying technologies use cement, etc. during construction, emissions are yet to be brought to zero. Each result is contrasted with coal and fossil gas at 820 and 490 g CO2 eq/kWh, there is a social debate about nuclear power. Proponents, such as the World Nuclear Association and Environmentalists for Nuclear Energy, contend that nuclear power is a safe, opponents, such as Greenpeace International and NIRS, contend that nuclear power poses many threats to people and the environment. These include the Chernobyl disaster which occurred in 1986, the Fukushima Daiichi nuclear disaster, there have also been some nuclear submarine accidents. Energy production from coal, petroleum, natural gas and hydroelectricity has caused a number of fatalities per unit of energy generated due to air pollution. In 2015, Ten new reactors were connected to the grid, seven reactors were permanently shut down. 441 reactors had a net capacity of 382,855 megawatts of electricity. 67 new nuclear reactors were under construction, Most of the new activity is in China where there is an urgent need to control pollution from coal plants. In October 2016, Watts Bar 2 became the first new United States reactor to enter commercial operation since 1996. The same year, his doctoral student James Chadwick discovered the neutron, further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new, transuranic element and they determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces, contradicting Fermi. Numerous scientists, including Leó Szilárd, who was one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. In the United States, where Fermi and Szilárd had both emigrated, this led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2,1942. In 1945, the pocketbook The Atomic Age heralded the untapped atomic power in everyday objects and depicted a future where fossil fuels would go unused. One science writer, David Dietz, wrote that instead of filling the gas tank of a car two or three times a week, people travel for a year on a pellet of atomic energy the size of a vitamin pill. The United Kingdom, Canada, and the USSR proceeded over the course of the late 1940s, electricity was generated for the first time by a nuclear reactor on December 20,1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW. Work was also researched in the US on nuclear marine propulsion

6.
Uranium
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Uranium is a chemical element with symbol U and atomic number 92. It is a metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons, Uranium is weakly radioactive because all its isotopes are unstable. The most common isotopes in natural uranium are uranium-238 and uranium-235, Uranium has the highest atomic weight of the primordially occurring elements. Its density is about 70% higher than that of lead, and it occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite. In nature, uranium is found as uranium-238, uranium-235, Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years, many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 is the naturally occurring fissile isotope, which makes it widely used in nuclear power plants. However, because of the amounts found in nature, uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor, another fissile isotope, uranium-233, can be produced from natural thorium and is also important in nuclear technology. In sufficient concentration, these maintain a sustained nuclear chain reaction. This generates the heat in nuclear reactors, and produces the fissile material for nuclear weapons. Depleted uranium is used in kinetic energy penetrators and armor plating, Uranium is used as a colorant in uranium glass, producing lemon yellow to green colors. Uranium glass fluoresces green in ultraviolet light and it was also used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, eugène-Melchior Péligot was the first person to isolate the metal and its radioactive properties were discovered in 1896 by Henri Becquerel. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of weapons that used uranium metal. The security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 is a concern for public health. When refined, uranium is a white, weakly radioactive metal

7.
Heat
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In physics, heat is the amount of energy flowing from one body to another spontaneously due to their temperature difference, or by any means other than through work or the transfer of matter. Thus, energy exchanged as heat during a process changes the energy of each body by equal. The sign of the quantity of heat can indicate the direction of the transfer, for example from system A to system B, negation indicates energy flowing in the opposite direction. While heat flows spontaneously from hot to cold, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems, conversely, a heat engine reduces an existing temperature difference to do work on another system. Heat is a consequence of the motion of particles. When heat is transferred between two objects or systems, the energy of the object or systems particles increases, as this occurs, the arrangement between particles becomes more and more disordered. In other words, heat is related to the concept of entropy, historically, many energy units for measurement of heat have been used. The standards-based unit in the International System of Units is the joule, Heat is measured by its effect on the states of interacting bodies, for example, by the amount of ice melted or a change in temperature. The quantification of heat via the change of a body is called calorimetry. In calorimetry, sensible heat is defined with respect to a specific chosen state variable of the system, sensible heat causes a change of the temperature of the system while leaving the chosen state variable unchanged. Heat transfer that occurs at a constant system temperature but changes the state variable is called latent heat with respect to the variable, for infinitesimal changes, the total incremental heat transfer is then the sum of the latent and sensible heat. Physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of many who began to build on the established idea that heat has something to do with matter in motion. This was the idea put forth by Benjamin Thompson in 1798. One of Maxwells recommended books was Heat as a Mode of Motion, Maxwell outlined four stipulations for the definition of heat, It is something which may be transferred from one body to another, according to the second law of thermodynamics. It is a quantity, and so can be treated mathematically. It cannot be treated as a substance, because it may be transformed into something that is not a material substance. Heat is one of the forms of energy and this was the way of the historical pioneers of thermodynamics. Maxwell writes that convection as such is not a purely thermal phenomenon, in thermodynamics, convection in general is regarded as transport of internal energy

8.
Electricity
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Electricity is the set of physical phenomena associated with the presence of electric charge. Although initially considered a separate to magnetism, since the development of Maxwells Equations both are recognized as part of a single phenomenon, electromagnetism. Various common phenomena are related to electricity, including lightning, static electricity, electric heating, electric discharges, in addition, electricity is at the heart of many modern technologies. The presence of a charge, which can be either positive or negative. On the other hand, the movement of charges, which is known as electric current. When a charge is placed in a location with non-zero electric field, the magnitude of this force is given by Coulombs Law. Thus, if that charge were to move, the field would be doing work on the electric charge. Electrical phenomena have been studied since antiquity, though progress in theoretical understanding remained slow until the seventeenth and eighteenth centuries. Even then, practical applications for electricity were few, and it would not be until the nineteenth century that engineers were able to put it to industrial and residential use. The rapid expansion in electrical technology at this time transformed industry, electricitys extraordinary versatility means it can be put to an almost limitless set of applications which include transport, heating, lighting, communications, and computation. Electrical power is now the backbone of modern industrial society, long before any knowledge of electricity existed, people were aware of shocks from electric fish. Ancient Egyptian texts dating from 2750 BCE referred to these fish as the Thunderer of the Nile, Electric fish were again reported millennia later by ancient Greek, Roman and Arabic naturalists and physicians. Patients suffering from such as gout or headache were directed to touch electric fish in the hope that the powerful jolt might cure them. Ancient cultures around the Mediterranean knew that certain objects, such as rods of amber, Thales was incorrect in believing the attraction was due to a magnetic effect, but later science would prove a link between magnetism and electricity. He coined the New Latin word electricus to refer to the property of attracting small objects after being rubbed and this association gave rise to the English words electric and electricity, which made their first appearance in print in Thomas Brownes Pseudodoxia Epidemica of 1646. Further work was conducted by Otto von Guericke, Robert Boyle, Stephen Gray, in the 18th century, Benjamin Franklin conducted extensive research in electricity, selling his possessions to fund his work. In June 1752 he is reputed to have attached a key to the bottom of a dampened kite string. A succession of jumping from the key to the back of his hand showed that lightning was indeed electrical in nature

9.
Electricity generation
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Electricity generation is the process of generating electric power from sources of primary energy. For electric utilities, it is the first process in the delivery of electricity to consumers, the other processes as transmission, distribution, energy storage and recovery using pumped-storage methods are normally carried out by the electric power industry. Other energy sources include solar photovoltaics and geothermal power, the fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday. This method is used today, electricity is generated by the movement of a loop of wire. Central power stations became practical with the development of alternating current power transmission, using power transformers to transmit power at high voltage. Electricity has been generated at central stations since 1882, the use of power-lines and power-poles have been significantly important in the distribution of electricity. There are seven fundamental methods of transforming other forms of energy into electrical energy. Static electricity, form the physical separation and transport of charge and it was the first form discovered and investigated, and the electrostatic generator is still used even in modern devices such as the Van de Graaff generator and MHD generators. In Electromagnetic induction, a generator, dynamo or alternator transforms kinetic energy into electricity. This is the most used form for generating electricity and is based on Faradays law and it can be experimented by rotating a magnet within closed loops of a conducting material. Almost all commercial electrical generation is done using electromagnetic induction, in mechanical energy forces a generator to rotate. Almost all electrical power on Earth is generated with a turbine, driven by wind, water, there are many different methods of developing mechanical energy, including heat engines, hydro, wind and tidal power. Most electric generation is driven by heat engines, the combustion of fossil fuels supplies most of the heat to these engines, with a significant fraction from nuclear fission and some from renewable sources. The modern steam turbine currently generates about 80% of the power in the world using a variety of heat sources. Power sources include, Steam Water is boiled by coal burned in a power plant. Nuclear fission heat created in a nuclear reactor creates steam, less than 15% of electricity is generated this way. Natural gas, turbines are directly by gases produced by combustion. Combined cycle are driven by steam and natural gas

10.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world

11.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity

12.
Gravity
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Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, on Earth, gravity gives weight to physical objects and causes the ocean tides. Gravity has a range, although its effects become increasingly weaker on farther objects. The most extreme example of this curvature of spacetime is a hole, from which nothing can escape once past its event horizon. More gravity results in time dilation, where time lapses more slowly at a lower gravitational potential. Gravity is the weakest of the four fundamental interactions of nature, the gravitational attraction is approximately 1038 times weaker than the strong force,1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, gravity has an influence on the behavior of subatomic particles. On the other hand, gravity is the dominant interaction at the macroscopic scale, for this reason, in part, pursuit of a theory of everything, the merging of the general theory of relativity and quantum mechanics into quantum gravity, has become an area of research. While the modern European thinkers are credited with development of gravitational theory, some of the earliest descriptions came from early mathematician-astronomers, such as Aryabhata, who had identified the force of gravity to explain why objects do not fall out when the Earth rotates. Later, the works of Brahmagupta referred to the presence of force, described it as an attractive force. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and this was a major departure from Aristotles belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere, galileos work set the stage for the formulation of Newtons theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. Newtons theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the position of the planet. A discrepancy in Mercurys orbit pointed out flaws in Newtons theory, the issue was resolved in 1915 by Albert Einsteins new theory of general relativity, which accounted for the small discrepancy in Mercurys orbit. The simplest way to test the equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the rate when other forces are negligible

A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial security devices issue a …

Smoke Detector COFEM with approved EN 54-7

Inside a basic ionization smoke detector. The black, round structure at the right is the ionization chamber. The white, round structure at the upper left is the piezoelectric horn that produces the alarm sound.

Heat is the amount of energy that transfers from a warmer object to a cooler one. More generally, heat arises from many …

The Sun and Earth form an ongoing example of a heating process. Some of the Sun's thermal radiation strikes and heats the Earth. Compared to the Sun, Earth has a much lower temperature and so sends far less thermal radiation back to the Sun. The heat of this process can be quantified by the net amount, and direction (Sun to Earth), of energy it transferred in a given period of time.

Rudolf Clausius

Joseph Black

A red-hot iron rod from which heat transfer to the surrounding environment will be primarily through radiation.

Benjamin Franklin conducted extensive research on electricity in the 18th century, as documented by Joseph Priestley (1767) History and Present Status of Electricity, with whom Franklin carried on extended correspondence.

Michael Faraday's discoveries formed the foundation of electric motor technology

Two-dimensional analogy of spacetime distortion generated by the mass of an object. Matter changes the geometry of spacetime, this (curved) geometry being interpreted as gravity. White lines do not represent the curvature of space but instead represent the coordinate system imposed on the curved spacetime, which would be rectilinear in a flat spacetime.

An initially-stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. This image spans half a second and was captured at 20 flashes per second.

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of …

A laser beam used for welding.

Laser beams in fog, reflected on a car windshield

A helium–neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The pink-orange glow running through the center of the tube is from the electric discharge which produces incoherent light, just as in a neon tube. This glowing plasma is excited and then acts as the gain medium through which the internal beam passes, as it is reflected between the two mirrors. Laser output through the front mirror can be seen to produce a tiny (about 1 mm in diameter) intense spot on the screen, to the right. Although it is a deep and pure red color, spots of laser light are so intense that cameras are typically overexposed and distort their color.

Radioactive decay (also known as nuclear decay or radioactivity) is the process by which an unstable atomic nucleus …

Pierre and Marie Curie in their Paris laboratory, before 1907

Taking an X-ray image with early Crookes tube apparatus in 1896. The Crookes tube is visible in the centre. The standing man is viewing his hand with a fluoroscope screen; this was a common way of setting up the tube. No precautions against radiation exposure are being taken; its hazards were not known at the time.

Gamma-ray energy spectrum of uranium ore (inset). Gamma-rays are emitted by decaying nuclides, and the gamma-ray energy can be used to characterize the decay (which nuclide is decaying to which). Here, using the gamma-ray spectrum, several nuclides that are typical of the decay chain of 238U have been identified: 226Ra, 214Pb, 214Bi.